1. Technical Field
This invention relates generally to a probe adapted to measure the dielectric properties of soil and other materials. More specifically, this invention relates to a probe for measuring the moisture content of soil or other medium.
2. Background Art
In the past, there have been a number of instruments used to measure the moisture content in soil so that farmers, ranchers, conservationists and the like could determine when to irrigate crops, plants, trees, etc. Early devices included taking bore samples of soil and placing the samples in devices that would measure the amount of moisture content in the soil. These devices generally required time-consuming oven-drying processes to determine the moisture content. The time delays, sometimes taking several days to characterize the moisture content of the soil, resulted in crops being either over- or under-irrigated for periods of time. As a result, crop damage or quality loss was a common occurrence.
Other soil moisture measuring devices, such as neutron source back scatter devices have also been used to measure soil moisture. These devices are bulky to transport to the field where soil measurements are typically taken and they rely on radioactive elements. These radioactive devices often are costly, require specialized personnel to operate, and have to be calibrated in the field prior to use at each measurement site.
In contrast to the prior art described above, more recent moisture measuring devices have been devised which operate on the principles of Time Domain Reflectrometry (TDR). Geologists and others have long recognized a relationship between the dielectric properties of soil, rock and other materials, and their moisture content. However, they initially lacked the instrumentation necessary to make full use of this knowledge. Time Domain Reflectometry, largely developed as the result of World War II radar research, offered a method to define these dielectric relationships. With the advent of commercial TDR research oscilloscopes in the early 1960s, it became feasible to test this new technology. Today, TDR technology is the xe2x80x9ccutting edgexe2x80x9d methodology for many diverse applications including the determination of basic soil water and material/water relationships.
TDR systems utilize the principle of TDR in order to convert the travel time of a broadband, electromagnetic pulse into volumetric water content. In practice these TDR systems generate a fast-rise pulse and send it at the speed of light down a transmission line consisting of at least two parallel wave guides or a coaxial arrangement of probes that are inserted or buried in the soil or other material to be measured. The velocity of propagation of the broadband pulse (often that incorporates frequencies that exceed 3 GHz) in soil is determined primarily by its water content. The pulse is reflected from the open ends of the wave guides/probes and returns along the original path. By microprocessor or other computing device, the travel time of the pulse is used to calculate the apparent dielectric constant of the soil. The actual digitized TDR wave form created as the pulse progresses down the wave guides can be displayed on a high resolution graphic LCD display for storage and interpretation. The actual time delay and correlated volumetric water content may also be digitally displayed on the screen.
TDR systems eliminate the need for using nuclear based instrumentation and the associated radiation, health and safety hazards. These systems eliminate site-specific calibration and the requirement for costly, specialized licensed personnel associated with neutron probes. They also provide auto logging capabilities that are not practical with nuclear techniques.
In the past, probes used with TDR systems have been manufactured having an inner conductive core that is surrounded by a dielectric material. This thin dielectric layer between the transmission element and the material being measured retains and reflects some of the energy along the transmission element as the pulse progresses from start to endpoint within the probe. This dielectric layer is key as it provides for a high coefficient of reflection that is necessary for the determination of the probe""s end points in highly conductive materials such as saline water, or conductive particle mixtures. This determination of these endpoints is used in the calculations to determine the apparent dielectric constant of the material being measured. For some measurements in high conductivity materials, end point reflections are attenuated to an insignificant level, undetectable by waveform analysis. In such cases, the high coefficient of reflection provided by the dielectric layer surrounding the inner conductive core is absolutely necessary to obtain accurate TDR measurements.
Although the dielectric layer provides for a high coefficient of refection, one problem with having the outer layer being made of soft or brittle dielectric materials is that it wears, scratches or breaks during multiple insertions into the ground or other material where the moisture is to be measured. This is particularly true when the probe is used to measure the moisture content of hard or abrasive soils, since the probes are typically repeatedly inserted in the soil to obtain the desired measurements. Additionally, the outer dielectric material is often difficult to manufacture such that an even layer is achieved. When the outer layer of dielectric is worn uneven, scratched or is unevenly manufactured the dielectric properties, and therefore the accuracy of the probe, is degraded.
Therefore, what is needed is a TDR probe that is immune to wear and scratching of the dielectric material coating the inner conductive element and that is easily manufactured to achieve reliable and consistent dielectric measurement results.
The system and process of the present invention satisfies all of the foregoing needs. The system and process provides a wave guide-like structure or probe (usually used in pairs or sets) that can be used for measuring the moisture content of soil or other materials. Probes according to the present invention, using the techniques described herein, can also be used in the following measurement areas:
(1) determination of concentrations of particles suspended in water and other liquids;
(2) determination of the amount of air entrapped in liquids, slurries and gels;
(3) determination of the liquid levels of immiscible fluids having differing dielectric character;
(4) determination of the proportional content in differing dielectrics soluble in water; and
(5) determination of the bulk content of the material being measured.
In the probe according to the present invention, the inner core of the probe is composed of an inner metallic element. This metallic element is a conductor that is used to transmit a broadband pulse. A dielectric liquid, solid or gel surrounds this inner conductive core, and assists in retaining broadband signal strength. The outer dielectric material is then encased in an outer shell that serves as a protective housing for the probe. This outer shell is preferably made of stainless steel, but can be made of other conductive materials as they serve to protect the inner dielectric material and metallic core. This combination of features allows a pulse transmitted through the probe to penetrate into the material being measured, while protecting the dielectric-coated transmission elements. Only the inner conductive elements (conductor and dielectric layer) are active and electrically connected to the components of the TDR system. Since the outer shell is not electrically connected in any manner to the transmission elements it simply acts as a tough and effective housing that protects a xe2x80x9cdielectrically-coatedxe2x80x9d transmission element. This invention combines the need for a dielectric coating on the transmission element and strong outer covering into a single assembled unit that provides both protection for multiple insertions into abrasive or in highly corrosive materials while allowing maximum signal retention for end point reflection determination.
The probe of the system and method according to the present invention, which is typically used as a pair or in sets, allows for the broadcasting of pulsed energy between the probes to migrate through the material being measured as well as along the dielectric-coated transmission elements of the probes. The net result is a much greater reflected energy from the pulse as it reaches the end points of the probes. This high coefficient of reflection from the pulse reaching the probe endpoint is instrumental in determining accurate and repeatable time determinations for speeds of propagation through the material being measured by the pair or set of probes. The probe according to the present invention is effective because it retains certain pulse energy and frequencies both in the continuous distributed dielectric coating and the transmission element making it an apparent continuous capacitive element to lower frequencies within the broadband pulse. As a result, more energy is stored in this dielectric layer and available to be reflected when the pulse reaches the endpoint of the probe.
One issue of note is that the travel time of a transmitted pulse using the encased probe of the present invention will be distorted because a portion of the pulse signal travels the probe interior while the greater portion travels more slowly between sets of probes (in the material being measured). This effect tends to shorten the travel times being measured when compared to probes without any dielectric coating. To adjust for this issue, a lookup table must be created that properly equates shortened travel times derived from the probe of the present invention to the actual dielectric constants of the materials being measured. Such a lookup table can be created by using the probe of the present invention to take measurements of travel times and speed of propagation of an electromagnetic pulse and associated reflection in various materials and then determining the moisture content, liquid levels, concentrations, and so on. Once this table is generated it can be used for future measurements to determine the moisture content, liquid levels, and concentrations that correspond to each range of subsequently-measured travel times.
The method of operation of the probe of present invention is relatively simple. A broadband pulse is transmitted from a pulse generator down coaxial transmission lines or cables to the probe or probes. The signal travels the length of the inner conducting element. In doing so, some of the pulse energy is broadcast through the dielectric layer, and then through the outer protective shell into the material being measured. Some of the energy is captured in the dielectric layer, increasing the speed of the signal as it is propagated down the inner conducting element. The remainder of the signal is broadcast through the dielectric and outer shell out into the material being measured. Due to an impedance mismatch where the coaxial cable and the probes are joined, a downgoing beginning reference point, a TDR wave form feature, is produced. The beginning reference point provides a starting point for measuring the accumulated time delay as the electronic pulse continues to travel through the probe. An ending reflection point, a TDR wave form feature, is created as the electronic pulse reaches the end of the probe and transmits into the surrounding soil. Upon reaching the probe end point there is ample signal strength to indicate a reflected end point. This reflected end point is the result of a complex interaction between a portion of the broadband pulse frequencies and energies that travel through the inner conducting elements of each individual probe, and the portion of the broadband pulse frequencies and energies that travel between two or more of the probes as well as through the material being measured. The apparent dielectric of the soil may be ascertained having determined the accumulated delay time, and in turn one can ascertain the moisture content of the soil using the apparent dielectric value. Once the travel times are known the moisture content of the material can be determined by utilizing a look up table common in TDR techniques as was discussed above.
In many cases it is necessary to make measurements in high loss materials that are abrasive and/or corrosive. The low loss probe of the present invention has shown an ability to both provide a tough, protective, outer surface to protect the inner workings of the probe for multiple insertions in abrasive materials such as soils, while providing ample signal for noticeable and detectable endpoints in high loss materials being measured. High loss materials being measured can be soils (intermixed materials), slurries, or gels, whose water contains high salinity content, or electrically conductive particles that diminish the broadcasting ability of the broadband signal between probe pairs being used to make material measurements.
The probe of the present invention can have different configurations. For instance, one embodiment of the probe is rod-shaped. This rod-shaped probe has a rod-shaped conducting element. Surrounding this conducting element is a layer of dielectric material. An outer protective shell surrounds the dielectric layer and conducting element in a concentric manner. The rod-shaped probe embodiment can have a cone-shaped pointed tipped to make it easier to insert it into soil or other material being measured. Additionally, this rod-shaped probe can be of different lengths and diameters.
Another embodiment of the probe of the present invention entails a disc-shaped probe. A pair of these disc-shaped probes can be configured to align along a common axis, such that a circular hole through each disc is aligned in parallel and can be mounted on some sort of axle or chassis that can be attached to a tractor or other vehicle. The discs can then be forced into the ground so that measurements can be taken while the vehicle is moving. The inner core of each disc-shaped probe is a conducting element. A dielectric material is sandwiched between the conducting element and an outer shell.
It has been found that more precise readings can be obtained using TDR techniques when a longer effective conductor length exists (resulting in longer travel times of the aforementioned electromagnetic pulse) in the probes being used to take the measurement. Another embodiment of the present invention uses this principle to employ a coil-shaped conductor within a rod-shaped probe. In this embodiment, a coil-shaped conductor is potted within a dielectric material. An outer protective shell surrounds the combination of the coil-shaped conductor and the dielectric material. Through the use of the coil-shaped conductor the effective length of the conductor is increased over what can typically be found in a probe of similar length that employs a straight conductive member, thus improving the accuracy of the probe(s).
In yet another embodiment of the present invention, a pair of probes is concentrically or coaxially-configured. The inner probe is rod-shaped, having an inner conducting element that is covered with a dielectric layer. An outer protective shell encases both the inner conducting element and the dielectric layer of this probe. A ring-shaped probe surrounds the rod-shaped probe, with the two probes being concentrically located, but separate. The ring-shaped probe also has an inner conducting element that is surrounded by a dielectric layer. An outer protective shell encases both the inner conductor and the dielectric layer of the ring-shaped probe. In use, the material to be measured in situated between the two probes such that the material is lodged between the two probes.
In another embodiment of the probe of the present invention, the probe is plate-shaped. This probe has a plate-shaped inner conductive element that is surrounded by a dielectric layer. The dielectric layer is encased in a protective outer shell. In use, two plate-shaped probes can be configured such that they are parallel to each other and the material to be measured flows between them, along the larger surfaces of the probes. However, these probes can also be configured such that they are closer to each other at one end and are further apart at the other end. This configuration might typically be used in a mixer or bulk transfer systems where material flows between the probes and differing densities of the material to be measured may make it difficult to get accurate readings. Angling the probes relative to each other in this manner increases the density of the material to be measured as it is compressed between the probes where the probes are least separated in distance. This increased density of the measured material allows for more accurate and consistent measurements of the dielectric properties of the materials. It should also be noted that the plate-shaped probes in this embodiment do not necessarily have to be flat. These probes could be curved if desired. Additionally, these plate-shaped probes do not have to be rectangular in shape. For instance, they could be circular, triangular or configured in any type of geometric cross-sectional shape.
In yet another embodiment of the probe of the present invention, a longer conductor is embedded within a plate-shaped probe. The longer conductor is a snake-like element encased in a dielectric material. The inner conductor and dielectric material are covered by an outer protective shell. Again, as discussed in the embodiment above, these probes can be angled toward each other and can also be of different shapes (e.g., rectangular, circular, triangular, octagonal, etc.)
The probe of the present invention can be made of various materials. The inner conductive element is made of a conductive material, typically a metal such as stainless steel or copper, for example. The dielectric layer that surrounds the inner conductive element of the probe of the present invention can take the form of solid, liquid or gel. Typically the dielectric layer is, for example, a hardenable epoxy of known dielectric constant. The outer protective shell is typically made of stainless steel, but could equally well be made of other suitable material.